1. Field of the Invention
The present invention relates to an apparatus for measuring a magnetic field by means of a superconductive magneto-resistive element which can detect a weak magnetic field by utilizing the magneto-resistive effect of a ceramic superconductor.
2. Description of the Prior Art
Before the description proceeds, it is noted that the word "resistance" means "electric resistance" and "magneto-resistive element" means "the element of which electric resistance of the element is changed corresponding to the magnetic field applied to the element".
Conventionally, semiconductors, magnetic materials or other devices with magneto-resistive effects have been utilized for detecting or measuring a magnetism. A magneto-resistive element such as a ferromagnetic metal, Fe-Ni (Permalloy) or Co-Ni shows resistance increase along a quadratic curve with respect to the strength of an applied magnetic field, so that a bridge circuit is ordinarily used for detecting the magnetic field.
Furthermore, such a magnetic sensor is being developed that can detect a weak magnetic field by utilizing the magneto-resistive effect of a ceramic superconductor in which the grain boundaries are weakly joined to one another.
The conventional magneto-resistive element using a magnetic material or the like is small in resistance variation, particularly, with respect to near zero magnetic field. Accordingly, a bias magnetic field by a permanent magnet or an electromagnet is applied to this element to use a large variation in resistance with respect to variation in the magnetic field strength for measurement. However, such a device has suffered poor measuring accuracy and sensitivity.
Furthermore, there is another type of magneto-resistive element using a newly-developed ceramic superconductor in which the grain boundaries are weakly joined, for enabling to measure even a weak magnetic field. FIG. 1 shows one of the characteristics of the superconductive magneto-resistive element. FIG. 1 is a graph in which the axis of abscissa represents a magnetic field strength H when a constant current I is fed to the magneto-resistive element, to which a magnetic field is applied; and the axis of ordinate represents voltage V developed in the magneto-resistive element by the above mentioned current and magnetic field. As apparent from the graph, a superconductive state of the superconductive magneto-resistive element is collapsed by application of a weak magnetic field, and it abruptly shows an electrical resistance. After that, as the magnetic field strength is increased, the resistance of the element is radically increased. In addition, the larger the current fed to the element becomes, the smaller a critical magnetic field Hc inducing resistance becomes, while the larger the resistance increase to the magnetic field becomes.
As mentioned above, the magnetic sensor using a ceramic superconductor can detect and measure a weak magnetic field, however has a problem that it is difficult to adjust current to a value at which the magnetic sensor can have characteristics suitable for an applied magnetic field. Furthermore, the magnetic sensor using a permanent magnet or electromagnet for enhancing the sensitivity has a problem that it is difficult to adjust a bias magnetic field to an optimum value.